The present invention relates to a method of inexpensively producing a neodymium-iron-boron permanent magnet alloy.
It is known to produce a rare earth-iron-boron permanent magnet, a new high-performance permanent magnet, by a method comprising using as starting materials rare earth metals, electrolytic iron, electrolytic cobalt, pure boron or ferroboron, and melting, pulverizing, pressing in a magnetic field and sintering (Japanese Pat. Laid-Open No. 59-215460). This conventional melting method, however, is costly because it uses expensive rare earth metals.
Recently a so-called reduction method has been proposed as an alternative to the melting method. For instance, Japanese Pat. Laid-Open No. 59-219404 discloses a method of producing a rare earth-iron-boron permanent magnet alloy powder by a reduction reaction which comprises the steps of mixing rare earth oxide powder, iron powder, ferroboron powder and cobalt powder with metal calcium or calcium hydride in an amount of 2-4 times (by weight) as much as stoichiometrically necessary for the reduction of the rare earth oxide powder, heating them in an inert gas atmosphere at 900.degree.-1200.degree. C., and washing the resulting reaction product in water to remove reaction by-products.
And Japanese Pat. Laid-Open No. 59-177346 discloses the use of a flux in the reduction method to lower the viscosity of a eutectic melt.
The above conventional reduction method (Japanese Pat. Laid-Open No. 59-219404) needs water washing for a long period of time to remove calcium oxide, a by-product of the reduction reaction. For the rare earth element-iron-boron alloys rich in iron, extreme oxidation takes place in this water washing step, increasing the oxygen content in the resulting alloys. Thus, it is difficult to stably obtain magnet alloys with good magnetic properties.
At the same time it is extremely difficult to completely remove calcium oxide by water washing, and the remaining calcium oxide lowers the sinterability of the permanent magnet alloys in the sintering step, thereby deteriorating the magnetic properties of the resulting permanent magnets.
And the conventional reduction method using a flux (Japanese Pat. Laid-Open No. 59-177346) did not succeed in reducing the calcium content in the resulting alloys to an acceptable level.
Recently, a proposal was made to produce a neodymium-iron or neodymium-iron-boron mother alloy for use in producing neodymium-iron-boron magnet alloys by charging starting materials of neodymium fluoride, calcium, iron and if necessary, boron oxide together with calcium chloride as a flux to an iron container, and melting them at 750.degree.-1000.degree. C. in a non-oxidizing atmosphere to reduce the neodymium fluoride to neodymium (Japanese Pat. Laid-Open No. 61-84340). This method, however, fails to provide a neodymium-iron-boron alloy containing an extremely small amounts of calcium. Accordingly, the magnets produced from these mother alloys inevitably contain relatively large amounts of calcium which acts to decrease magnetic properties. In addition, since the mother alloys have compositions far different from those of the final permanent magnets, an additional melting step is needed to change the compositions of the alloys to the desired ones by the addition of iron and boron.